<!DOCTYPE html>
<html>
<head>
	<title>flex 4*4</title>
	<style type="text/css">
		.box{
			width: 100%;
			display: flex;
			flex-wrap: wrap;
			height: 400px;
			border-top: 1px solid #000;
			border-right: 1px solid #000;
		}
		.box .item{
			box-sizing: border-box;
			flex:0 0 25%;
			border-left:1px solid #000;
			border-bottom:1px solid #000;

		}
	</style>
</head>
<body>
	<div class="box">
		<div class="item"></div>
		<div class="item"></div>
		<div class="item"></div>
		<div class="item"></div>
		<div class="item"></div>
		<div class="item"></div>
		<div class="item"></div>
		<div class="item"></div>
		<div class="item"></div>
		<div class="item"></div>
		<div class="item"></div>
		<div class="item"></div>
		<div class="item"></div>
		<div class="item"></div>
		<div class="item"></div>
		<div class="item"></div>
	</div>

</body>
</html>